Thyroidal regulation of substrate kinetics properties of cytochrome oxidase in rat liver mitochondria

Effects of thyroidectomy (T_x) and subsequent treatment with 3,5,3′-triiodothyronine (T₃), and combined treatment (T _R) with T₃ + thyroxine (T₄) on substrate kinetics properties of cytochrome oxidase of rat liver mitochondria were examined. T_x resulted in lowering of cytochromes content with decrease in the enzyme activity, and K_m and V_max. T ₃ and T_R regimens restored the cytochromes contents and the V_max values to normal. In control, T₃ and T _R groups the enzyme activity resolved in two kinetic components; in T_x group three kinetic components were evident. The K_m values for all components decreased significantly in the experimental groups with concomitant increase in catalytic efficiency, K_cat/K _m. Significant alterations in the contents of total phospholipid and of cholesterol were noted while the changes in the phospholipids composition were only of restricted nature. Regression analysis revealed that total phospholipid, cholesterol and phosphatidylcholine, phosphatidylethanolamine play significant role in fine tuning the enzyme activity.